Understanding the Child Tax Credit

I have quite a few questions and things to confirm.

1. Have we always gotten this child tax credit, just at the refund time? 

2. Has the amount changed? What is the total amount per child? 

3. I currently have 3 kids under 5 and I am receiving $300 per kid. How long will I get these payments? Until December? Until September? Until March of next year?

4. Is there a portion of the tax credit that I will still see during the refund time or am I getting the full disbursement monthly now?

5. I assume if my normal tax return is say 7500, but my total child tax credit is 9000 (3000 per child), then I would owe taxes of 1500 during tax filing time, is that correct?